DTM knows all states of the loading page. A build in feature for measuring the page load time would be a simplification and better then using any javascript custom code.
In general default usage of the "PerformanceTiming" functionality in JS would be great, to have out of the box performance measurement for not only page load time, but also rendering time, connection and look time etc...
I understand why this would be helpful, but our general advice to customers not to use your tag manager to monitor your tag manager. We suggest a more objective 3rd party, regardless of the tag manager you use. I hope that helps.
I agree with the don't measure DTM from DTM side. Use a RUM or similar tool for measuring page load. However, giving us an easy way to pull a page load time into analytics can help us figure out what page load times convert client's relative to other page load times as seen by DTM.
